Organic electronic devices are devices that carry out an electrical operation using an organic material. They are expected to offer advantages such as low energy consumption, low cost, and flexibility and they are receiving attention as a technology for replacing conventional silicon-based inorganic semiconductors.
Organic EL devices, as a subset of organic electronic devices, are being considered, for example, for application as a large-area solid-state light source to replace incandescent lamps and gas-filled lamps. In addition, in the flat panel display (FPD) sector they are also being considered as a front-running self-emissive display technology to replace liquid crystal displays (LCD) and their development as commercial products is underway.
Organic EL devices can be generally classified into two categories based on the materials used and the film production method: small molecule organic EL devices and polymer organic EL devices. The organic material is composed of a polymer material in the case of polymer organic EL devices, and polymer organic EL devices are essential devices for the large-screen organic EL displays of the future since they support easier film formation, e.g., by printing or inkjet, than is the case for the small molecule organic EL devices, which require film formation in a vacuum system.
While to date both small molecule organic EL devices and polymer organic EL devices have been the subject of strong research efforts, their low emission efficiency and short device lifetime are still major problems in both instances. The formation of small molecule organic EL devices in multiple layers has been carried out as one approach for solving these problems.
An example of a multilayer organic EL device is shown in FIG. 1. In FIG. 1, the layer that carries out light emission is designated as the light-emitting layer 1, and when other layers are present the layer in contact with the anode 2 is designated as the hole injection layer 3 and the layer in contact with the cathode 4 is designated as the electron injection layer 5. When another layer is present between the light-emitting layer 1 and the hole injection layer 3, this layer is designated as a hole transport layer 6; when another layer is present between the light-emitting layer 1 and the electron injection layer 5, this layer is designated as an electron transport layer 7. Reference number 8 in FIG. 1 refers to a substrate.
Since in the case of small molecule organic EL devices film formation is carried out by vapor deposition procedures, multilayering can be easily accomplished by carrying out vapor deposition while successively changing the compounds used. In the case of polymer organic EL devices, on the other hand, film formation is carried out using a wet process, such as printing or inkjet, and in order to elaborate multiple layers a procedure is therefore required in which the previously formed layers are not altered during the production of a new layer.
In practice, almost all polymer organic EL devices are devices that have the following two-layer structure: a hole injection layer comprising polythiophene:polystyrenesulfonic acid (PEDOT:PSS) and formed using a water-based dispersion, and a light-emitting layer formed using an aromatic organic solvent such as toluene. Fabrication of the two-layer structure in this instance is made possible by the fact that the PEDOT:PSS layer is not soluble in toluene.
The difficulty that has been encountered in elaborating even more layers in the case of polymer organic EL devices is due to the dissolution of the lower layers when layering is carried out using similar solvents. In order to respond to this problem, a device has been proposed that has a three-layer structure that uses compounds with substantially different solubilities (refer, for example, to Nonpatent Reference 1). In addition, a device has also been reported that has a three-layer structure that contains a hole transport layer that utilizes a photocuring reaction (refer, for example, to Nonpatent Reference 2). A device has also been reported that has a three-layer structure that has a hole transport layer that utilizes a crosslinking reaction by siloxane compounds (refer, for example, to Nonpatent Reference 3). While these are important processes, they have not been free of problems; for example, solubility considerations place limitations on the materials that can be used, while the siloxane compounds are unstable to atmospheric moisture. In addition, in all instances the device characteristics have not been satisfactory.    Patent Reference 1: U.S. Pat. No. 4,539,507    Patent Reference 2: U.S. Pat. No. 5,151,629    Patent Reference 3: International Publication WO 90/13148 Pamphlet    Patent Reference 4: European Patent Publication EP 0 443 861 A    Nonpatent Reference 1: Y. Goto, T. Hayashida, M. Noto, IDW '04 Proceedings of the 11th International Display Workshop, 1343-1346 (2004)    Nonpatent Reference 2: Kengo HIROSE, Daisuke KUMAKI, Nobuaki KOIKE, Akira KURIYAMA, Seiichiro IKEHATA, and Shizuo TOKITO, 53rd Meeting of the Japan Society of Applied Physics and Related Societies, 26p-ZK-4 (2006)    Nonpatent Reference 3: H. Yan, P. Lee, N. R. Armstrong, A. Graham, G. A. Evmenenko, P. Dutta, T. J. Marks, J. Am. Chem. Soc., 127, 3172-4183 (2005)    Nonpatent Reference 4: T. Yamamoto, Bull. Chem. Soc. Jap., Volume 51, Number 7, p. 2091 (1978)    Nonpatent Reference 5: M. Zembayashi, Tet. Lett., Volume 47, p. 4089 (1977)    Nonpatent Reference 6: Synthetic Communications, Volume 11, Number 7, p. 513 (1981)